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A spherometer is used for the measurement of thickness of thin plates and the radius of curvature of a spherical surface. It works on the principle of a micrometer screw gauge.
It consists of a metal frame supported on three fixed legs of equal length. The end of three legs are pointed and form the three corners of an equilateral triangle. A screw whose pitch is usually 1 mm carries a circular disc at its top, is so supported that the tip of this screw is at the center of the triangle formed by the tips of the three legs as shown in Fig. A small main scale is fixed vertically at one end of the frame very close to circular disc and is usually graduated in millimeters. The reading of the circular scale is taken against the inner edge of main scale.
Pitch and Least Count: When the central screw of spherometer is given one complete rotation, it advances or recedes by 1 mm, (the pitch of the screw is one millimeter). Now, one rotation means turning of disc through 100 division on disc only, will move the screw by ( 1 x 1/100) mm = 0.01 mm. This is the least count of the spherometer. It means that the instrument can measure up to 0.01 mm.
Thus in general
Least Count (LC) =
pitch of the screw
no. of divisions on circular scale
Zero Error: When the three legs and tip of the central screw of spherometer just touch a plane surface, the zero line of circular scale and zero of the main scale should coincide. But it is often found particularly with old instruments that the zero of circular scale if ahead or behind the edge of main scale when tips of the legs and screw lie in the same plane. This initial error is called zero error. The algebraic sign of this error depends upon the direction in which a subsequent measurement is to be made. In laboratories, the determination of zero error is done by noting the reading of spherometer on base plate
A spherometer is used for the measurement of thickness of thin plates and the radius of curvature of a spherical surface. It works on the principle of a micrometer screw gauge.
It consists of a metal frame supported on three fixed legs of equal length. The end of three legs are pointed and form the three corners of an equilateral triangle. A screw whose pitch is usually 1 mm carries a circular disc at its top, is so supported that the tip of this screw is at the center of the triangle formed by the tips of the three legs as shown in Fig. A small main scale is fixed vertically at one end of the frame very close to circular disc and is usually graduated in millimeters. The reading of the circular scale is taken against the inner edge of main scale.
Pitch and Least Count: When the central screw of spherometer is given one complete rotation, it advances or recedes by 1 mm, (the pitch of the screw is one millimeter). Now, one rotation means turning of disc through 100 division on disc only, will move the screw by ( 1 x 1/100) mm = 0.01 mm. This is the least count of the spherometer. It means that the instrument can measure up to 0.01 mm.
Thus in general
Least Count (LC) =
pitch of the screw
no. of divisions on circular scale
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